1. Field of the Invention
This invention relates generally to oxygen generating compositions, and more particularly concerns improved low temperature sensitivity oxygen generating compositions including alkali metal silicate, stannate, titanate or zirconate as a reaction rate and rheology modifier and chlorine suppresser.
2. Description of Related Art
Chemical oxygen generators are typically used in situations requiring the generation of supplemental oxygen, such as in aviation and in submarines, for example, and in other similar settings where it is useful to furnish a convenient and reliable supply of oxygen gas of breathable quality. Chemical oxygen generating compositions based upon the decomposition of alkali metal chlorates or perchlorates have long been used as an emergency source of breathable oxygen in passenger aircraft, for example. Oxygen generating compositions utilizing alkali metal chlorates or perchlorates are disclosed in U.S. Pat. Nos. 5,198,147; 5,279,761; and 5,298,187; each of which are incorporated herein by reference.
An aircraft oxygen generator is commonly preprogrammed according to the descent profile of a given type of airplane, and must meet the minimum oxygen flow requirement at all times during a descent. When the oxygen generating reaction is initiated at one end of the core, the reaction front propagates along the longitudinal axis toward the other end of the core as oxygen is generated. Ideally, the reaction zone moves at a steady, repeatable rate governed by the amounts of fuel and catalyst incorporated at a given point along the length of the core. However, oxygen gas formed inside a chemical oxygen generation core or candle must develop sufficient pressure to escape from the core. This effect can cause an uneven and erratic flow of oxygen from an activated oxygen generation core.
In order to meet minimum flow requirements despite such variations in oxygen flow from the core, excess weight of the chemical oxygen generating composition is commonly used. Heavier cores also typically need to be used to insure the cores have sufficient duration, because relatively large cavities can develop during operation of such oxygen generating candles, resulting in irregular oxygen flow when oxygen generating candles made with such compositions are operated at high temperatures. Melting of the oxygen generating core under such conditions can also make the core vulnerable to high intensity vibrations. To minimize unnecessary weight, particularly in aircraft, it would be desirable to provide oxygen generating compositions that can facilitate the flow of oxygen from an activated oxygen generation core. It would also be desirable to provide oxygen generating compositions that are substantially free from carbon and organic contamination that can result in unacceptable levels of carbon monoxide or carbon dioxide contamination in the oxygen produced, that have lower sensitivity to environmental temperatures, and that are structurally more robust to withstand high levels of vibration during operation.
When expended chemical core residues are visually examined, several conditions can be observed which indicate behavior that occurred during the oxygen generating reaction. When the oxygen evolved at a steady and smooth rate, the pores left in the residue are typically small and uniform. The presence of large cavities typically indicate the formation of very large bubbles associated with very large bursts of oxygen release. Such large bubbles tend to perturb heat transfer into other regions of the core, and can result in a large burst of oxygen release follow by a temporary sharp decline or dip in oxygen evolution.
Gross physical distortion in the shape of the residue, relative to the shape of the unreacted core, can be evidence of a very runny reaction zone that can result in possible mechanical failure of the core in the event of exposure of the core to severe vibration during operation of the oxygen generator. On the other hand, relatively uniform, laminar patterns of pores in the residue is suggestive of a well ordered reaction zone. The presence of irregular swirls in the residue can indicate that the reaction zone was severely disturbed and may have mechanically collapsed, which can also be correlated with an irregular flow of oxygen.
The various reaction behaviors that are observable in the residues of oxygen generation cores are related to the melt properties of the chemical core. The reaction temperature is approximately 500.degree. C. or higher inside the operating chemical core. Because sodium chlorate melts at about 265.degree. C., during operation of the oxygen generator, sodium chlorate can melt in an unconstrained manner and form puddles that can cause the core to collapse. Unconstrained melting, puddling, and collapsing of the core can result in a disorganized, irregular reaction front and an irregular oxygen generation rate, causing variation in performance from core to core, and can make the oxygen generation rate and the rate at which the reaction zone moves more temperature dependent, particularly in that the oxygen generation rate becomes much lower at lower temperatures due to bursts and dips. Solid phase decomposition of the oxygen generating reaction mixture can also cause the undecomposed portion of the core to crack, resulting in an erratic oxygen generation rate. This phenomenon is particularly likely at lower environmental temperatures. Since a minimum oxygen flow and a minimum duration are required at all operating temperatures, a heavier conventional oxygen generating core is commonly needed to insure the oxygen flow curve does not dip below the customer specification for operation under cold conditions and that the duration is longer than the specification for hot conditions.
In addition, when chemical cores melt in an unconstrained way, the melted material can come in contact with the oxygen generator housing, resulting in hot spots on the generator wall, which can result in temperatures that exceed applicable performance specifications. The duration of oxygen generation can also be much shorter at higher temperatures due to a poorly organized reaction zone, which can have a larger reacting volume than expected. Oxygen generating compositions are also commonly required to function within a wide range of environmental temperatures, as low as -30.degree. and as high as 60.degree. C., for example. Since the rate of decomposition of sodium chlorate is temperature dependent, an excess weight of the chemical oxygen generating composition is commonly used in order to insure that both required minimum flow rates in colder temperatures and minimum duration specifications at high temperatures are met. It would be desirable to provide oxygen generating compositions that do not require increasing the core weight, and can provide a more uniform rate of oxygen generation over an operating range of temperatures.
There is a need to provide oxygen generating cores that do not melt in an unconstrained manner to form puddles, and that retain their structural integrity and shape during operation of the oxygen generator, allowing reduction or elimination of preformed insulation layers used to increase the mechanical integrity of the operating core. There is also a need to reduce the probability of a localized high temperature spot on the generator wall, to lower the maximum wall temperature during operation. It would further be desirable to provide oxygen generating compositions that produce smoother oxygen flow curves and have lower temperature sensitivity. The present invention meets these needs.